Ring laser gyroscopes measure rate by detecting phase shifts (called Sagnac phase shifts) in counter-rotating beams of light. The light rotates within a cavity which includes a lasing element. It is important that the cavity length be maintained to a certain degree of accuracy, preferably, within 0.001 .lambda., or less. In one configuration, this is effected by mounting a mirror to a piezoelectric (PZT) element. Path length adjustment is accomplished by applying a DC voltage to the PZT material.
Additionally, it is desired to apply an AC modulation, or modulation dither, to the cavity length control. This is done in order to search for a resonant mode of operation. This dither modulation is accomplished by adding an AC component to the voltage applied to the PZT material.
The expansion of the ring laser's frame due to temperature deviation is relatively small (2.7 .lambda.), compared to the expansion of a prior art cavity length control housing assembly (8.2 .lambda.), so that a cavity length control design with minimal temperature expansion is desirable. If the expansion of the cavity length control housing assembly is reduced, the total range required is also reduced. The tilt of the cavity length control is due to inconsistencies in the expansion of the PZT material and the adhesive bond. Hysteresis problems are due to the residual deformation of the PZT/housing adhesive bond over voltage and temperature.
It is therefore desired to provide a cavity length control arrangement whereby deviation as a result of thermal changes is minimized, thereby increasing the frequency stability of the cavity.